DNA methylation has been implicated in the control of a number of cellular processes in eukaryotes, including transcription, genomic imprinting, developmental regulation, DNA repair and mutagenesis. Cytosine-5-methyltransferases (m5C-Mtases) catalyze the transfer of a methyl group from S-adenosyl-L-methionine (AdoMet) to the C5 position of cytosine. It is put forward in this proposal that the exact mechanism of DNA recognition binding and methylation by the HHaI DNA cytosine-5- methyltransferase, be investigated. The exact elements involved in DNA recognition and binding will be probed by determining the crystal structures of HhaI/DNA complexes in which binding but no methylation takes place. The structures of complexes of the native enzyme with-either hemi- or fully methylated DNA will be elucidated in order to understand how DNA returns the target base to the helix, after methylation has taken place. This information will certainly provide insight into how the DNA methylation process might be controlled in biological systems.